This invention relates to an X-ray apparatus equipped with a rotary anode type X-ray tube. In a conventional X-ray apparatus, a rotary anode type X-ray tube, the anode of which revolves so that the electrons will not strike the anode at one point only, is used to control the temperature rise of the anode. The life of this rotary type X-ray tube is determined by the temperature rise of the inside of the tube, the temperature rise of the anode, and the life of the bearing which holds the anode, allowing it to revolve.
The temperature of the anode rises as the tube current increases, and this results in an increase in the number of electrons emitted from the cathode. The temperature of the anode can be lowered by increasing its rotation speed and substantially expanding the area where the electrons strike, i.e., the target area. If the tube current is small, the temperature rise of the anode is correspondingly small. In such a case, the rotary anode is rotated at a low speed, allowing for the life of the bearing for the motor. Thus, by adjusting the rotating speed of the rotary anode according to the tube current, the life of the X-ray tube can be extended.
To change the rotating speed of the anode, the motor speed is changed. For example, if the tube current is greater than a predetermined value, the motor speed is set to high speed, e.g. 180 rps (revolution per second). When it is below that value, the motor speed is set to low speed, e.g. 60 rps.
In the past, the temperature rise of the anode has been controlled, as described above, by changing the revolution speed of the anode in accordance with the tube current. However, the cause of temperature rise of the anode is not confined solely to the tube current. The temperature also varies according to the tube voltage, radiation time, and size of the focal point. Having not taken these other factors into consideration, the revolution speed has therefore simply been increased when the tube current was large, so that the life of the bearing has consequently been short.